The Wasatch Mountains in Utah are famous for having "The Biggest Snow on Earth". Snow hunters seeking world-class skiing and snowboarding contribute more than a billion dollars annually to the economy. The Snowmelt also provides most of the water for populations that grow rapidly along the Wasatch Front, including Salt Lake City. Understanding what controls the timing and magnitude of the thaw is critical for Utah.
It is more complicated than heating the air temperature; the energy of the sun and the long hours of daylight in the spring are the main drivers of the thaw. Like wearing a black shirt on a hot day, anything that darkens the surface of the snow – like dust – absorbs more sunlight and speeds up the melting. As humans continue to alter landscapes, dust is more likely to blow on nearby peaks. However, scientists are just beginning to understand the impact of dust on snow.
A new study from the University of Utah analyzed the impacts of dust deposition on an alpine study area in Alta, Utah, in the Wasatch Mountains. For the first time, researchers measured dust in the air and snow simultaneously. They discovered that a single dust storm on April 13, 2017 deposited half of all the station dust. Extra sunlight absorbed by the dust darkened the surface of the snow and led to the melting of snow a week before.
Using computer simulations, the team modeled where the dust originated. They discovered that first, ahead of the storm, came the dust of the south, but then moved west. West winds brought dust from the "hot spots" on the dry lake bed of the Great Salt Lake, a relatively new source of dust due to the lake's low historic levels.
"What's important about the Great Salt Lake is that there are no rights to water, no policy to maintain lake levels. As the lake declines, dust events are projected to become more frequent, "said McKenzie Skiles, assistant professor of geography at U and lead author of the study. "Anything that has an impact on snowmelt could have economic and hydrological consequences. And now one of the regions of dust origin is next door. Could we do something about it by passing a policy that maintains a minimum lake level? "
The study published online December 21 in the journal Environmental Research Charters.
Snowmelt in Wasatch
Skiles and his team observed five dust events during the spring of 2017 but focused on a single storm on April 13 because they deposited the largest amount of dust and because the dust seemed to come from the dry bed of Great Salt Lake. The team collected data at the Atwater Study Plot, near Alta Ski Resort in Alta, Utah. They collected data in three ways. First, they sampled the size and number of airborne particles. Second, they excavated wells to analyze the properties of snow and samples of dust concentrations. Third, they used computer simulations to estimate where the dust came from and where it should go. They were confident in the simulations because they captured the patterns in air and snow samples.
To measure how the dust hit the snow, Skiles calculated the difference in energy absorption between the dust-blackened snow and the same snow if it remained dust-free. The equation incorporates the properties of snow, such as the size of the snow grains, the snow depth, the depth and the mixture of aerosols. The overall impact of the powder was to accelerate the melting by 25%.
They found that most of the dust was deposited about an hour after the storm, in the so-called "post-frontal" winds. Sources such as the Great Deseret Salt Lake were the largest emitters of dust; the dust from the critical points of the dry lake bed represented about 10% of the deposited powder. However, computational simulations suggest that much of the powder blew north of the study plot. With no snow observations in the area, the researchers were unable to verify the largest dust deposition, but they assumed the impact was probably higher in North Wasatch.
"In most people, dust is a natural aerosol. But the magnitude and frequency of airborne dust is affected by human activity, changing landscapes makes the dust more prone to being absorbed by the wind, "said Skiles. "We know that since the colonization of the west, the amount of dust in the air has increased. And at the same time, because of upstream water abstractions, lake levels are also declining, exposing even more dust. "
Dust is a global problem
In October, Skiles co-authored an article that reviewed the literature on the growing global issue of "light-absorbing particles" on snow in the journal Nature. The climate science community has recognized the impact of aerosols that are clearly linked to human activity, such as soot, but still need to consider other particles that make the snow darker and accelerate the melting.
"Overall, snow is declining and it's not just from warmer weather – it's more complicated than that – the snow is also getting darker," Skiles said. "We know that in some places, aerosols are affecting water resources and are having this long-term climate impact. We also know that deposition levels are unlikely to decrease in the future. Although we still do not understand the exact magnitude of the impact, we know that dust deserves more attention. "
Skiles studied the dust in the snow in the Colorado Rockies, but wants to continue looking at other mountainous areas.
"I'm interested in looking at Wyoming, Montana and here in Utah because we need the regional perspective – we know that dust has a dramatic impact on the melting of the ice in Colorado, but how is the impact elsewhere? If the dust is not so important in those regions, so what is controlling the patterns of snow change? "she asked.